1. Field of the Invention
The invention relates to a mandrel lubricant for the production of seamless tubes, especially for the coating of mandrels that have passed through a cooling bath after the rolling operation and are being prepared for the next rolling operation.
2. Background Information
In modern tube rolling mills, e.g., in continuous tube trains (MPM-trains), the shaping of the seamless tubes takes place in the main process step by rolling of a prefabricated ingot heated to 1200.degree. to 1300.degree. C. over a mandrel which is mounted on a mandrel rod. After the rolling operation, the mandrel or the mandrel rod is removed from the tube round and fed to a cooling bath where the mandrel or the mandrel rod is cooled down from about 150.degree. to 350.degree. C. to a temperature of about 60.degree. to 100.degree. C., and in this way, is prepared for the next rolling operation. A lubrication composition is also included in this preparation of the mandrel or mandrel rod after the cooling bath. This lubrication composition is essential to assure an optimal "sliding" of the ingot on the mandrel rod during the rolling operation and is one of the factors decisive for the later quality of the tube, i.e., for the condition of the inner surface of the tube.
Generally, this lubrication composition was made with graphite-containing oils. By burnoff of oil on contact of the mandrel with the hot ingot, a very strong smoke development occurred that, because of the toxic components of the smoke, led to an unacceptable pollution of the surrounding area and environment. On the other hand, this uncontrolled burnoff caused by the ingot during the rolling operation led to damage to the lubricant coating on the mandrel and possibly to damage to the inner surfaces of the tubes.
Then, in German Patent No. 2450716, especially in Example 5, it was proposed to use a high temperature lubricant essentially consisting of graphite and an alkylene polymer. This lubricant was applied immediately after the rolling operation of the still uncooled mandrel, whereupon a dry and partially water-resistant lubricant film formed. The mandrel so treated was then able to be fed to the cooling bath and later, without relubrication, again to the rolling process. But it has turned out that by the mechanical stress on the mandrel in transport, by the cooling bath and by the action of the water, the lubricating film is often damaged which in turn has a negative effect on the quality of the tube.
Since in modern rolling trains the lubrication composition of the mandrels takes place after the cooling bath, it was then attempted, corresponding to the lubrication composition with graphite-containing oils, to apply the lubricant formulation of German Patent No. 2450716 to mandrels that have already passed the cooling bath. As required by the process, the time between application of the lubricant to the 60.degree. to 100.degree. C. mandrel up to the rolling operation is limited to a maximum of 5 seconds.
As the present inventors' comparison tests show (see examples), even the formation of a dry lubricant film with the lubricants according to German Patent No. 2450716 takes at least 15 seconds. As a result, the same effect is observed as in the treatment of the mandrel with graphite-containing oils when the treated mandrel comes in contact with the hot ingot. Moreover, the polymeric components are subjected to pyrolysis which leads to damage to the lubricant film during the rolling operation and, thus, finally adversely affects the quality of the tubes.